1. Field of the Invention
This invention relates to a screwdriver (mainly such as a power screwdriver) for use in tightening a screw into, e.g., a piece of plasterboard that is used such as on a ceiling and a wall.
2. Description of the Related Art
A screwdriver such as a power screwdriver is primarily intended for use on a fastened material such as a piece of plasterboard. The screwdriver includes an axially movable and adjustable stopper disposed on part of a unit body because a screw must be driven into the fastened material at a certain depth during screw tightening. The screwdriver further includes a screw-tightened depth adjuster for releasing a clutch when the unit body is moved to a position where the stopper abuts the fastened member during the screw tightening. The screwdriver yet further includes a mechanism for transmitting power through a one-way clutch when a motor is reversely revolved in order to back off the screw.
One such example of a prior art screwdriver as disclosed in Japanese Patent Application Laid-Out No. 8-267367 will now be described with reference to FIG. 1.
In FIG. 1, a threadingly engaged stopper sleeve 3 is shown mounted on a housing 2 at a distal end thereof. The stopper sleeve 3 is relatively rotated with respect to the housing 2, thereby adjusting a relative distance therebetween. The adjusted relative distance provides an adjusted screw-fed distance (or adjusted screw-driven depth).
In the housing 2, a pinion 19 on an output shaft of a motor (not shown) is engaged with a gear 5 that is circumferentially mounted on a driving shaft member 4. The driving shaft member 4 and the gear 5 are rotated as a one-piece component in a rotational direction of the driving shaft member 4, and motor rotation is transmitted to the driving shaft member 4 through the gear 5.
Referring to FIG. 2, a compression spring or a clutch spring 6 including axially extending upper and lower ends is shown coiled in a counterclockwise direction.
An output shaft member 7 includes engagement balls 18 that allow the output shaft member 7 to receive a screw-driving bit 10 in a jointly rotatable manner. A metal section 8 supports the output shaft member 7 in a pivotable and axially movable manner, but limits movement of the output shaft member 7 toward the screw-driving bit 10. Part of the output shaft member 7 is positioned within a one-way clutch 16. The one-way clutch 16 is retained within a driving shaft sleeve 4a of the driving shaft member 4. The output shaft member 7 is axially movable within the one-way clutch 16, while the one-way clutch 16 permits the output shaft member 7 to be pivoted in a single direction (in a direction in which a screw is tightened) with reference to the one-way clutch 16.
A non-rotatable, but axially movable locking member 12 is positioned against the stopper sleeve 3 at an end thereof toward the driving shaft member 4. The locking member 12 is always urged toward the stopper sleeve 3 by means of a compression spring 11. The stopper sleeve 3 and the locking member 12 includes convex and concave claws, respectively, which claws are in constant mesh with one another at a position where the locking member 12 abuts the stopper sleeve 3. When the claw of the stopper sleeve 3 is held in mesh with that of the locking member 12, then the stopper sleeve 3 is locked against rotation with respect to the housing 2, and is further immovable in an axial direction of the stopper sleeve 3. The screw-tightened depth can be adjusted by the stopper sleeve 3 being relatively rotated in relation to the housing 2 when an operator disengages the claw of the stopper sleeve 3 from that of the locking member 12 by manually sliding the locking member 12 toward the driving shaft member 4 against the spring force of the compression spring 11.
In the driving shaft member 4, an axially movable umbrella-like member 13 is positioned against the output shaft member 7 at an end thereof in a direction opposite to the screw-driving bit 10. A radially movable ball 9 within a through-hole 4b of the driving shaft sleeve 4a is seated against the umbrella-like member 13 along the outer circumference thereof or rather a tapered portion 13a. A spring 15 constantly urges the umbrella-like member 13 toward the output shaft member 7.
In FIG. 1, the pressing force of the spring 15 causes the output shaft member 7 having the screw-driving bit 10 disposed thereon to be moved in a downward direction of FIG. 1 through the umbrella-like member 13, and a distal end of the bit 10 remains projecting beyond that of the stopper sleeve 3.
When driving the screw is started in such a state, then output shaft member 7 with the screw-driving bit 10 thereon is axially moved in an upward direction of FIG. 1 through the umbrella-like member 13 against the pressing force of the spring 15.
The tapered portion 13a is slanted at an angle that permits an outer diameter thereof to decrease with an increase in distance between the tapered portion 13a and the output shaft member 7. Such a construction allows ball 9 on the tapered portion 13a to protrude beyond the driving shaft sleeve 4a along the outer circumference thereof in response to the axial movement of the output shaft member 7 with the screw-driving bit 10.
An output shaft sleeve 7a having the same outer diameter as that of the driving shaft sleeve 4a is disposed on the output shaft member 7. The output shaft sleeve 7a includes a fixing portion 14a and a protruding portion 14b. The fixing portion 14a secures an end of the clutch spring 6 that extends about the output shaft sleeve 7a and driving shaft sleeve 4a along the respective outer circumferences thereof. The protruding portion 14b extends beyond the output shaft sleeve 7a along the outer circumference thereof, and abuts an end surface of the clutch spring 6, thereby restricting the clutch spring 6 in axial position thereof. As illustrated in FIGS. 2 and 3, the clutch spring 6 has the bent end secured by the fixing portion 14a, and is thereby disposed in an axially immovable fashion.
The opposite end 6a of the clutch spring 6 toward the driving shaft member 4 is a free end, and the clutch spring end 6a is axially moved in union with the axially moved output shaft member 7 having the bit 10 carried thereon.
When the output shaft member 7 with the screw-driving bit 10 is axially moved against the spring force of the spring 15 during screw tightening, then the clutch spring end 6a is axially moved in union with the output shaft member 7, and is then positioned on the ball 9 along the circumference thereof. The umbrella-like member 13 is axially moved in response to the movement of the output shaft member 7 with the screw-driving bit 10, and the tapered portion 13a causes the ball 9 to protrude beyond the driving shaft sleeve 4a along the outer circumference thereof. As a result, the clutch spring end 6a can be brought into contact with the ball 9 in a rotational direction of the driving shaft member 4, and a motor rotational force is transmitted to the clutch spring 6 through the gear 5, driving shaft member 4, and ball 9.
When a motor is normally revolved, then the clutch spring 6 is deformed in a direction in which an outer diameter of the clutch spring 6 is reduced, and is thereby wound around the driving shaft sleeve 7a as well as the output shaft sleeve 4a along the respective outer circumferences thereof, thereby rotating the output shaft sleeve 4a jointly with the driving shaft sleeve 7a. As a result, the revolving force of the driving shaft member 4 is transmitted to the output shaft member 7 through the clutch spring 6 in order to revolve the screw-driving bit 10, thereby tightening the screw.
When the screw is further driven, then an end surface of the stopper sleeve 3 is brought into contact with the fastened member, and the pressing force of the spring 15 causes the output shaft member 7 as well as the screw-driving bit 10 to be axially moved toward the distal end of the screw-driving bit 10 in response to further screw tightening. When the output shaft member 7 with the screw-driving bit 10 thereon is moved toward the distal end of the screw-driving bit 10 by a certain amount, then the ball 9 in abutment with the tapered portion 13a is retracted away from the circumference of the driving shaft sleeve 4a, which otherwise would remain protruding beyond the driving shaft sleeve 4a along the circumference thereof. As a result, the clutch spring 6 is unwound and then released from the sleeves 4a and 7a, and the output shaft member 7 with the screw-driving bit 10 is isolated from the revolving force, thereby completing the screw tightening.
In the prior art screwdriver as discussed above, the ball 9 and the clutch spring end 6a are brought into contact with one another in the revolving direction of the driving shaft member 4 during normal rotation of the motor that runs in order to drive the screw, and the clutch spring 6 is deformed in a direction in which an inner diameter thereof is reduced, thereby rotating the output shaft member 7 in union with the driving shaft member 4. As a result, the revolving force of the driving shaft member 4 is transmitted to the screw-driving bit 10 as well as the output shaft member 7. When the motor is reversely rotated in order to back off the screw, then the revolving force of the driving shaft member 4 is transmitted to the screw-driving bit 10 as well as the output shaft member 7 through the one-way clutch 16. However, there exists a drawback to the prior art screwdriver. More specifically, when the one-way clutch 16 is idled, then the ball 9 is brought into contact with the clutch spring end 6a as illustrated in FIG. 5 during movement of the output shaft member 7 in a direction in which the output shaft member 7 resists the pressing force of the spring 15. When the ball 9 collides with the clutch spring 6, then the clutch spring 6 experiences a load that acts in a direction as shown by an arrow in FIG. 5. As a result, the clutch spring 6 is deformed in a direction in which then inner diameter thereof expands as seen from FIG. 6, and the clutch spring 6 is immediately plastically deformed or otherwise broken. Consequently, the screwdriver is mal-functioned or otherwise rendered inoperative.
The one-way clutch 16 tends to be idled when the output shaft member 7 is axially moved with respect to the one-way clutch 16 upon motor start-up.
When the ball 9 is driven into contact with the clutch spring end 6a after idling of the one-way clutch 16 during screw loosening as previously discussed, then the clutch spring 6 is deformed in a direction in which the clutch spring 6 expands toward any space within the housing 2. Such deformation reduces an axial dimension of the clutch spring 6, and the clutch spring end 6a is disengaged from the ball 9. However, a large volume of torque must be exerted on the clutch spring 6 during such disengagement, and the clutch spring 6 is immediately plastically deformed or otherwise broken. As a result, the screwdriver is either brought out of normal service or rendered inoperative.
In view of the above, an object of the present invention is to provide a longer life screwdriver including a clutch spring resistant to plastic deformation and breakage when a ball abuts a clutch spring end after idling of a one-way clutch during screw loosening.
The above object is achieved by disengagement means for releasing engagement between the clutch spring and the ball during reverse motor rotation.
The above object is accomplished by a slanted surface on the clutch spring at the clutch spring end where the clutch spring is positioned against the ball during reverse motor rotation.
The above object is attainable by a through-hole configured to allow the ball to be moved away from an immovable shaft member when the ball is forced against the clutch spring during reverse motor rotation, part of the clutch spring being secured to the immovable shaft member.